This invention relates to methods and apparatus for reducing corrosion in porous refractory linings, and more particularly to methods and apparatus for reducing corrosion in refractory linings which contain chromia.
Iron alloys, including steel, can be produced by a number of different processes, including direct steelmaking. In the direct steelmaking process, concentated ore is placed in a furnace which operates at temperatures on the order of between about 1600.degree. C. and about 1800.degree. C., considerably above the melting point of the ore. The molten metal is contained in a vessel which includes a refractory lining on the inside of the vessel walls. As the molten metal is processed, undesired matter rises to the top of the molten metal and forms slag. The slag also includes matter added intentionally to remove impurities from the ore. The slag prevents the usable product beneath the slag from oxidizing, but the slag itself is an undesirable byproduct of the process and is eventually removed and discarded.
In direct steelmaking processes, concentrated iron ore in the form of hematite (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4) or the like is transformed through chemical reactions into wustite (FeO). The FeO becomes substantially pure iron (Fe) through further processing. Direct steelmaking processes have advantages over some other steelmaking processes because the capital costs of production are relatively low. However, the FeO is highly corrosive and dissolves most of the materials which are commonly used in refractory linings.
In direct steelmaking processes, some of the FeO does not undergo further reactions, but rises to the top of the molten metal and resides in the slag. In general, in direct steelmaking processes, the slag contains about 25 to 35 percent FeO. Temperatures of between about 1600.degree. C. and about 1800.degree. C., and such high iron oxide levels, create severe conditions which are very corrosive and destroy refractory lining materials. Thus, there is a need for refractory linings for vessels used in direct steelmaking processes which are resistant to corrosion when in contact with FeO at high temperatures, or processes which protect refractory linings under such conditions.
Some coal gasifiers operate at lower temperatures and lower FeO concentrations than the direct steelmaking processes just discussed. To resist chromiamagnesia corrosion, spinels containing about 80 percent chromium oxide (Cr.sub.2 O.sub.3) have been used in the refractory lining materials of the coal gasifiers.
Chromium is a multivalent metal. Chromium in the +3 valence state (Cr.sup.+3) is only slightly soluble by corrosive materials such as FeO, while chromium in the +2 valence state (Cr.sup.+2) is much more soluble and therefore vulnerable to corrosive chemicals. The valence state of chromium in refractory linings is determined by the temperature and the oxygen partial pressure at the lining.
Oxygen partial pressure is related to both the total gas pressure on a surface, and the concentration of oxygen at the surface. In the atmosphere, for example, air is about 80 percent nitrogen and about 19 percent oxygen, with traces of other elements, of course. The total pressure of the air is 1 atmosphere, and the ambient oxygen partial pressure is approximately 0.2 atmospheres.
The oxygen partial pressure in coal gasifiers is between about 10.sup.-10 and 10.sup.-8 atmospheres. At those pressures, the chromium in the refractory lining is maintained in the +3 valence state, which is only slightly soluble and is quite resistant to corrosion by FeO.
The refractory lining materials used in the coal gasifiers could be used in the direct steelmaking processes just discussed, but the oxygen partial pressure is several orders of magnitude lower in the direct steelmaking processes than it is in the coal gasifiers, and the chromium in the spinels will change to a +2 valence state. Such a change in valence state is undesirable because it results in a higher corrosion rate of the lining. Thus, there is a need for methods and apparatus for maintaining multivalent metal cations used in refractory linings in the most chemically resistant valence state. There is also a need for methods and apparatus for maintaining chromium used in refractory linings for metal producing processes having iron oxide as a byproduct in the +3 valence state.
Accordingly, one object of this invention is to provide new and improved methods and apparatus for resisting corrosion in refractory linings used in metal making processes.
Another object is to provide new and improved methods and apparatus for maintaining multivalent metal cations used in refractory linings in the most chemically resistant valence state.
Still another object is to provide new and improved methods and apparatus for maintaining chromium in the +3 valence state when used in refractory linings in direct steelmaking processes which operate between about 1600.degree. C. and about 1800.degree. C., and have FeO as a byproduct.